1. Field of the Invention
The invention relates to an image projector, and more particularly to an image projector including a liquid crystal display panel for receiving flux of light emitted from a light source located at the rear thereof, and projecting images in front thereof
2. Description of the Related Art
A conventional image projector is illustrated in FIG. 1. In the illustrated image projector, a light source 9 located at a focus of a reflection mirror 12 emits non-directional flux of light. The emitted flux of light is reflected at an inner surface of the reflection mirror 12 towards first to third image display units 2a, 2b and 2c. Specifically, a portion of flux of light is introduced into the first and second image display units 2a and 2b through an ordinary optical path 13, and a remainder of flux of light is introduced into the third image display unit 2c through a relay optical path 14.
For instance, each of the first to third image display units 2a to 2c is comprised of a liquid crystal display panel.
The flux of light passes through fly eye lenses 8a, 8b and a condensing lens 6 to thereby be diverged into some portions, and the thus diverged flux of light, that is, images of the fly eye lenses 8a and 8b are converged into a point.
Thus, the flux of light having reached the first to third image display units 2a, 2b, and 2c can have high brightness and uniformity in brightness in a screen. Since the flux of light emitted from the light source 9 has completely no polarizing components, polarizing components which could not pass through the first to third image display units 2a to 2c are turned into heat, which might cause a damage to the image display units 2a to 2c. To prevent the first to third image display units 2a to 2c from being damaged, a filter 7 is positioned between the fly eye lenses 8a, 8b and the condensing lens 6 to thereby remove polarizing components which are harmful to the first to third image display units 2a to 2c.
As an alternative, there may be used a part for separating polarizing components, converting a phase of the separated polarizing components, and re-combining the separated polarizing components with one another, in order to utilize polarizing components usually not used.
The flux of light emitted from the light source 9 is separated into three primary color components, that is, red (R), green (G), and blue (B), by means of first and second dichroic mirrors 11a and 11b. The thus separated red, green, and blue color components pass through the first, second, and third image display units 2a, 2b, and 2c, respectively. The first to third image display units 2a to 2c form images based on the red, green, and blue color components. A cross dichroic prism 1 combines the thus formed images into full-color image. The thus formed full-color image is projected onto a screen (not illustrated) through a projection lens 3 located adjacent to the cross dichroic prism 1.
The cross dichroic prism 1 has four outer surfaces. The projection lens 3 is positioned facing one of the outer surfaces of the cross dichroic prism 1, and the first to third image display units 2a to 2c are positioned facing the remaining outer surfaces, respectively, to thereby equalize a focal distance of the projection lens 3 to focal distances of the first to third image display units 2a to 2c.
Since the first and second image display units 2a and 2b are positioned relative to the first dichroic mirror 11a in mirror image relation, portions of the flux of light separated by the first dichroic mirror 11a run the same optical path length to the first and second image display units 2a and 2b, respectively.
On the other hand, a portion of the flux of light having passed through the second dichroic mirror 11b and directing to the third image display unit 2c has to run a longer optical path length than the optical path length for a portion of the flux of light to run towards the first or second image display unit 2a or 2b.
The fly eye lens 8a, 8b and the condensing lens 6 focus images of small lenses constituting the fly eye lenses 8a and 8b on the first and second image display units 2a and 2b. Accordingly, the optical path 14 to the third image display unit 2c has a focus offset from a focus of the optical path 13 to the first and second image display units 2a and 2b by a difference in an optical path length between the optical path 14 and the optical path 13. As a result, a portion of the flux of light having passed through the optical path 14 does not match in size to a screen of the third image display unit 2c, resulting in failure of uniform, efficient illumination.
To overcome this problem, first and second relay lenses 4 and 5 in an extension of the optical path 14 to the third image display unit 2c. The first and second relay lenses 4 and 5 enlarge and project the flux of light again, ensuring compensation for a width of the flux of light to thereby entirely cover the third image display unit 2c with the flux of light.
However, addition of the first and second relay lenses 4 and 5 causes another problem of a difference in optical characteristic between an ordinary optical path and a relay optical path. Herein, the term "an ordinary optical path" indicates an optical path in which flux of light reaches an image display unit without passing through a relay lens, and the term "a relay optical path" indicates an optical path in which flux of light reaches an image display unit via a relay lens. The above-mentioned problem is originated from a difference in brightness distribution between brightness of flux of light having passed the ordinary optical path and brightness of flux of light having passed the relay optical path.
Specifically, the flux of light emitted from the light source 6 is designed to be focused on an entrance pupil of the projection lens 3 by means of the condensing lens 6 in the first and second image display units 2a and 2b. On the other hand, since it is necessary to design the third image display unit 2c larger in size, the flux of light having passed the relay optical path in which the first and second relay lenses 4 and 5 are provided are not focused on an entrance pupil of the projection lens 3 in many cases.
In addition, aberration of the first and second relay lenses 4 and 5 causes the projection lens 3 to have a different tendency of focusing flux of light having passed the relay optical path from a tendency of focusing flux of light having passed the ordinary optical path.
Hence, only the third image display unit 2c is accompanied with a problem that uniformity in brightness of a screen on which images are focused is not ensured, resulting in non-uniform illumination for combined images comprising images having passed the relay optical path and images having passed the ordinary optical path.
As illustrated in FIG. 9, the first and second image display units 2a and 2b receiving flux of light having passed the ordinary optical path can have a brightness distribution where high brightness is found in almost entire area.
On the other hand, as illustrated in FIG. 10, the third image display unit 2c receiving flux of light having passed the relay optical path has a brightness distribution where a central area has a lower brightness than a brightness of an area around the central area, because of the above-mentioned aberration, a difference in optical length between the ordinary optical path and the relay optical path, and a difference in a position as to where images are focused on the projection lens 3.
Accordingly, if images are combined without any compensation, there would occur a difference in a brightness distribution in each color on a screen. As a result, three major colors, red, green and blue, are combined in unbalanced fashion, which would cause a problem that uniformity in color is not ensured in color combination on a screen.
It would be possible to focus flux of light on a location close to an entrance pupil by shortening a focal distance of each of the relay lenses 4 and 5. However, if a focal distance of each of the first and second relay lenses 4 and 5 is shortened, display magnification of the third image display unit 2c would be varied. This causes a problem that a display area to which images are focused might be too large or too small relative to a screen of the third image display unit 2c, resulting in shortage in brightness and uniformity in focusing images.
In order to overcome the above-mentioned problem, Japanese Unexamined Patent Publication No. 7-199181 (hereinafter, referred to as "first prior art") has suggested an image projector.
The first prior art includes a small-sized metal halide lamp having electrodes spaced away from each other by a distance in the range of 2.5 mm to 3.5 mm. Hence, the image projector as the first prior art can be small-sized, and also can have a bright screen.
In addition, since the first prior art includes a liquid crystal panel having a diagonal length of 2 inches or smaller, or having an area of about 1240 square millimeter or smaller, it is possible to reduce deterioration in a light-focusing rate on a panel. This ensures that sufficiently bright images can be focused on a screen.
However, the first prior art is accompanied with problems that it is quite difficult to space the electrodes from each other by a desired distance, and that it is also difficult to have uniform brightness on a screen.
Japanese Unexamined Patent Publication 9-113994 (hereinafter, referred to as "second prior art") has suggested a liquid crystal projector. The second prior art has almost the same object as that of the above-mentioned first prior art.
The liquid crystal projector suggested in the second prior art is comprised of a paraboloid mirror reflecting flux of light emitted from a light source, and turning the flux of light into parallel lights, an integrator lens including first and second convex lens groups each comprised of a plurality of convex lenses to which the parallel lights enter, an optical separator for separating the parallel lights having passed the integrator lens into lights having three major colors, first to third liquid crystal panels for optically modulating lights having been separated into three major colors by the optical separator, a dichroic prism for combining lights optically modulated by the first to third liquid crystal panels, and a projection lens for projecting the thus combined lights.
This liquid crystal projector is characterized by that optical lengths between the integrator lens and each of the first to third liquid crystal panels are designed equal to each other.
However, the second prior art is accompanied with problems that an arrangement of equalizing the optical lengths between the integrator lens and the liquid crystal panels restricts designability of other elements as to where they should be positioned, and that it is quite difficult to obtain uniform brightness on a screen similarly to the above-mentioned first prior art.
Japanese Unexamined Patent Publication No. 7-301778 has suggested an image projector including a projection lens comprised of a rear projection lens and a front projection lens, and means for combining hue, positioned between the rear and front projection lenses.
In accordance with the suggested image projector, parallel flux of light emitted from objects located on three display panels passes through parallel plates or a cross dichroic mirror, there does not occur coma-aberration. Hence, images are correctly focused on a screen.
Japanese Unexamined Patent Publication No. 8-304739 has suggested a polarizing illuminator including a light source emitting randomly polarized lights, a first lens plate comprising a plurality of rectangular condensing lenses for condensing lights emitted from the light source to thereby form a plurality of secondary light source images, and a second lens plate located in the vicinity of a location where the secondary light source images are formed.
In accordance with the suggested polarizing illuminator, polarized lights are separated when the first lens plate forms the minute secondary light source images. Hence, is would be possible to prevent spatial expansion of an optical path, which would be caused by separation of polarized lights. As a result, the polarizing illuminator can be fabricated in a smaller size.
Japanese Unexamined Patent Publication No. 9-138369 has suggested an image projector including a projection lens, and an optical system for combining colors. The projection lens is comprised of a first group of lenses and a second group of lenses, and the optical system is comprised of a transparent plate, dichroic mirrors, and a plain mirror. The second group of lenses is located between the optical system and polarizing beam splitters. A plane including a plane of the transparent plate, normal lines of dichroic mirrors, and an optical axis of the projection lens is designed perpendicular to a plane including normal lines of the polarizing beam splitters, and an optical axis of the projection lens.